In the production process of semiconductor devices, perfluorocarbons have been conventionally used as one of useful etching or cleaning gases.
On the other hand, to keep up with recent tendency toward higher performance, smaller size, higher density wiring and the like of electrical or electronic equipment, the electrode of a circuit substrate is becoming finer and in order to form a circuit pattern with higher precision by etching or the like, use of an extremely high-purity etching gas from which impurities are eliminated as much as possible is demanded. When the etching gas contains an impurity even if in a very small amount, this may cause generation of a large width line in the formation of a fine pattern or increase of defects in the product having a high density integrated circuit.
Also, the process of removing deposits using a cleaning gas must be performed to let residual impurities be reduced as much as possible in the production process of a semiconductor device after the cleaning so as to provide a high-purity and high-quality device. For this purpose, a high-purity cleaning gas containing substantially no impurity is demanded.
With respect to the process for producing perfluorocarbons, various methods have been heretofore proposed. For example, as for tetrafluoromethane, a method of reacting chlorotrifluoromethane with HF in the presence of a catalyst (see, JP-B-62-10211 (the term “JP-B” as used herein means an “examined Japanese patent publication”)) and a method of reacting dichlorodifluoromethane with HF in the presence of a catalyst (see, JP-B-42-3004) are known; and as for hexafluoroethane, an electrolytic fluorination method starting from ethane and/or ethylene, a pyrolysis method of thermally decomposing tetrafluoroethylene, and a method of fluorinating acetylene, ethylene, ethane or the like using a metal fluoride are known. Furthermore, a direct fluorination method of contacting hydrocarbon or hydrofluorocarbon with a fluorine gas is also known and examples thereof include a method of reacting trifluoromethane with a fluorine gas (see, GB 1,116,920), a method of reacting tetrafluoroethane with a fluorine gas (see, Japanese Patent 2,947,158), a method of reacting hexafluoropropylene with a fluorine gas (see, JP-B-62-61572), and a method of reacting carbon (C) with E2 in BrF3 or IF3 (see, JP-A-58-162536). Other than these, in the case of octafluoropropane as a perfluorocarbon having 3 carbon atoms, a direct fluorination method of reacting propane with a fluorine gas is known (see, EP 31519).
Of these various production processes, the direct fluorination method uses a fluorine gas having an extremely high reactivity and therefore, incurs dangers of bringing about explosion, corrosion or the like between the substrate organic compound and the fluorine gas and furthermore, dangers of causing side reactions such as abrupt reaction or explosion resulting from cleavage of C—C bond, polymerization, production of carbon (C), volume or the like due to generation of heat.
For example, in the case of synthesizing a perfluorocarbon by the direct fluorination method of reacting a linear hydrocarbon compound with a fluorine gas, the synthesis is accompanied with a very large heat of reaction as shown below.CH4+4F2→CF4+4HF  (Equation 1)                (ΔH=−479 kcal/mol)C2H6+6F2→C2F6+6HF  (Equation 2)        (ΔH=−690 kcal/mol)        
As such, in replacing one C—H bond by C—F bond, a heat of reaction of about −110 Kcal/mol is generated. In the direct fluorination method of reacting propane (C3H8) with a fluorine gas, ΔH is about −880 Kcal/mol.
In the case of starting from methane (Equation 1), 4 mol of fluorine gas is necessary per 1 mol of methane and in the case of starting from ethane (Equation 2), 6 mol of fluorine gas is necessary per 1 mol of ethane. In this way, as the number of hydrogen atoms in the substrate organic compound is larger or as the amount of fluorine used is larger, the heat of reaction becomes larger. In order to prevent the abrupt generation of heat of reaction in the direct fluorination method, there have been proposed, for example, a method of diluting the fluorine gas with another inert gas (e.g., nitrogen, helium), a method of diluting a substrate organic compound with another inert gas, a method of dissolving a substrate organic compound in a solvent inactive to fluorine in a low concentration, a method of performing the reaction in a low temperature region or a method of designing an apparatus such as jet reactor to allow the fluorine gas to gradually come into contact with the substrate organic compound when the reaction is performed in a gas phase.
The present inventors have already found that these problems encountered in the direct fluorination method can be solved by appropriately controlling the reaction conditions in the direct fluorination method and thereby, perfluorocarbons can be safely and economically produced in industry with good efficiency (Japanese Patent 3,067,633).
In the case of using the thus-obtained perfluorocarbons as a cleaning or etching gas in the process for producing a semiconductor device or the like, the perfluorocarbon must be free of various impurities as much as possible and have a high purity as described above. For the removal of impurities, separation by distillation or the like is usually used. Heretofore, perfluorocarbons having a fixed purity have been produced by the direct fluorination method where impurities are removed to a certain purity by combining, for example, purification of starting materials, and distillation and purification of the product.
In the process of studying the above-described method for obtaining perfluorocarbons by the direct fluorination method, the present inventors have found that some components remain as impurities even by performing high-precision distillation or the like and these residual impurities cannot be easily and effectively removed. By the analysis of these impurities, oxygen-containing compounds such as perfluorodimethyl ether, perfluorodimethyl peroxide and perfluoromethyl ethyl ether were detected. These oxygen-containing compounds were very difficult to remove because these formed an azeotropic composition or azeotrope-like mixture with perfluorocarbons. If such a perfluorocarbon is used as an etching or cleaning gas for the manufacture of a semiconductor device while allowing mixing of those oxygen-containing compounds in a high concentration, the requirement for formation of a very fine pattern may not be satisfied.
Accordingly, the present inventors have made extensive investigations to prevent the production of such oxygen-containing compounds, as a result, it has been found that these oxygen-containing compounds are originated in the oxygen gas contained in a slight amount in the reaction starting materials such as fluorine gas or hydrofluorocarbon, and when the oxygen gas content in the reaction starting materials is reduced to a specific amount or less while controlling the reaction conditions such as reaction temperature to fall within a certain range, the production of those oxygen-containing compounds can be effectively prevented. The present invention has been accomplished based on this finding. To the best knowledge of the present inventors, there has been not found a process for producing perfluorocarbons, which involves a technique of preventing the production of oxygen-containing compounds originated in the oxygen gas in starting materials.